It is common to provide a white light source by applying a yttrium aluminum garnet (YAG) phosphor layer over a blue light emitting diode (LED) die. The YAG phosphor emits a yellow-green light when energized by the blue light, and the combination of the blue light leaking through the phosphor layer and the yellow-green light creates white light. Many other colors can be formed in this way, and the invention is not limited to YAG phosphor or the use of LEDs.
FIG. 1 illustrates a conventional GaN-based LED die 10 that emits blue light. The die 10 may be a flip-chip die (both electrodes on bottom), a vertical die (one electrode on top and another on bottom), a lateral die (both electrodes on top), or other type of LED die. The LED semiconductor layers include an N-type layer 12, an active layer 14 (forming quantum wells), and a P-type layer 16. The active layer 14 emits blue light, in the example, such as light rays 18-21. These layers are epitaxially grown on a surface of a growth substrate (not shown), which may remain or is removed, depending on the type of die.
On top of the die 10 is deposited a phosphor layer 24. The phosphor layer 24 may be formed of a single phosphor or a combination of different phosphors, and may be a single layer or multiple layers. The phosphor layer 24 is typically formed of phosphor powder in a binder (e.g., silicone) or may a sintered tile of phosphor. If the phosphor layer 24 is in liquid form when deposited, it may be sprayed on for a wide-area application or deposited using a hollow needle for a die-by-die application. The density and thickness of the phosphor layer 24 must be carefully controlled, since the precise percentages of phosphor light and blue leakage light are required for generating a target color. In reality, the resulting color is variable due to the inability of the processes to consistently achieve the required amount of blue light leakage. The unevenness of the phosphor layer also adds to the variation in output color due to the variation in blue light leakage. Further, different LEDs, even from the same wafer, have slightly different peak wavelengths and flux, which change the required leakage properties of the phosphor layer 24.
In FIG. 1, phosphor particles 26A-26D in the layer 24 are shown energized by the blue light emitted by the active layer 14 and wavelength shift the light. The light emitted by the phosphor particles is generally isotropic, and the particular direction of a light ray emission is significant.
Light ray 18 is shown energizing particle 26A, and the secondary light emission is out of the top surface of the phosphor layer 24. Light ray 19 leaks through the phosphor layer 24. Light ray 20 is shown energizing particle 26B, and the secondary light emission is backscattered into the semiconductor layers and partially absorbed until it is somehow reflected back towards the top surface, such as by a bottom reflective electrode. Light ray 21 is shown energizing particle 26C, and the secondary light emission is backscattered by another particle 26D back into the semiconductor layers to be partially absorbed.
Any secondary light that is redirected back towards the semiconductor layers reduces the overall efficiency of the phosphor-converted LED (pcLED). Light extraction efficiency relates to the percentage of generated photons that escape the LED die 10. One goal in designing an LED die is to minimize light absorption so as to increase light extraction efficiency.
FIG. 2 illustrates the same phosphor layer 24 deposited on a transparent support substrate 30, which may be a flat plate or a lens, and the phosphor layer 24 is remote from the LED die 10. In this context, the term “remote” means that the phosphor layer 24 is separated from the semiconductor layers by some material or air. Light rays 32 and 33 are shown being emitted by the die 10. The same problems described with respect to FIG. 1 apply to FIG. 2.
Accordingly, as seen with respect to FIGS. 1 and 2, there is reduced efficiency caused by the backscattering, and the tolerances for the phosphor density and phosphor layer 24 thickness (affecting the blue light leakage) cause the resulting color to not be tightly controlled.
What is needed is a way to improve the color control of pcLEDs and improve the efficiency of such pcLEDs.